The present invention is related to Japanese patent application No. 2000-98140, filed Mar. 31, 2000; the contents of which are incorporated herein by reference.
The present invention relates to an AC generator for a vehicle which is driven by an internal combustion engine, and more particularly to an AC generator which is mounted on a vehicle such as a passenger car, a truck, and the like.
In recent years, vehicle bodes have been formed in slanted nose shapes to reduce air resistance acting on the vehicle body. Further, there is a demand for securing a sufficient residential space in the passenger compartment of the vehicle. This has reduced the engine room size, thereby limiting the space in which the AC generator for the vehicle can be installed. Also, in recent years, the engine speed has been reduced to save a fuel cost. Consequently, the rotational speed of the AC generator is reduced. On the other hand, demand for electric load has increased for safety control devices and the like. Thus, power-generating is increasingly demanded. Therefore, there is demand for a supply of compact, inexpensive, and high-power rotary electrical machine.
To comply with this demand, there has been proposed various devices, such as shown in FIG. 7, of Japanese Patent Application Laid-Open No.11-164499. Such a device allows the AC generator to output high power by setting the axial length of the iron core of the stator longer than the field coil of the rotor. Moreover, magnetic flux is injected directly into the iron core of the stator from the yoke to reduce the radial thickness of the claw-like magnetic pole and increase the winding area of the winding of the rotor by that much.
However, this construction has the following problems. Specifically, in a Lundel-type pole core, affected by an alternating magnetic flux applied by the stator, eddy currents are generated on the surface of the claw-like magnetic pole thereof. Because the claw-like magnetic pole is opposed to the stator which is heated to a high temperature by generated electric current, the claw-like magnetic pole is heated to an extremely high temperature by eddy current-caused self-heat development and heat transferred thereto from the stator. The claw-like magnetic pole is heated to an extremely high temperature at the central part of its peripheral surface to which the magnetic flux is collectively applied. A heat flow W generated there by the rise of temperature reaches the cylindrical part of the pole core as shown in FIG. 9, thus raising the temperature of the cylindrical part.
Referring to FIG. 8, in the construction disclosed in the Japanese Patent Application Laid-Open No.11-164499, the volume of the claw-like magnetic pole is small, and the area of the side surface S is also small. Thus, the claw-like magnetic pole has an eddy current-caused self-heat development owing to its reduced heat capacity. Further, the claw-like magnetic pole has a suppressed rotation-caused heat release. Therefore, the claw-like magnetic pole is heated to a very high temperature.
As a result, the heat flow increases due to the temperature rise of the claw-like magnetic pole and is transferred to the boss located at the central part of the iron core through the yoke part. Consequently, the central part of the rotor is heated to an extremely high temperature.
When the boss is heated to a very high temperature, the winding of the rotor, the resinous bobbin on which the winding is wound, and the iron core of the boss thermally expand. Because the coefficient of thermal expansion of the resin and the metal are different from each other, thermal stress is generated on the interface therebetween. In this case, as the temperature of the boss becomes higher, the thermal stress becomes increasingly large. Further, because the generator for a vehicle generates heat every time the vehicle is driven, the thermal stress is repeatedly generated on the boss, the bobbin, and the like and thereby causes fatigue.
Referring to FIG. 7, the gap A between the yoke part of the pole core and the bobbin on which the field coil is wound is filled with a resin-impregnated material. The difference between the thermal expansion of the iron core of the boss and the bobbin surrounding the iron core of the boss causes the resin-impregnated material filled in the gap A to crack or become damaged. Consequently, there is a reduction in the fixing strength at which the bobbin and the pole core are fixed to each other. Thus, when the rotor rotates, the bobbin rotates relative to the pole core, thus causing the coil to be cut and power generation to be stopped.
Therefore, it is necessary to enlarge the cooling fan to increase cooling wind. It is also necessary to reduce output current to restrain the claw-like magnetic pole from receiving heat from the stator, which reduces the intended effect of miniaturizing the generator and allowing it to have a high power output.
It is therefore an object of the present invention to provide a compact AC generator for a vehicle having a high power output and high reliability by minimizing a heat flow which is transferred to the central part of the pole core. Therefore, in a first aspect, the present invention provides an AC generator for a vehicle comprising a field rotor and a stator. The field rotor has a field coil and a Lundel-type iron core. The Lundel-type iron core has a cylindrical part on which the field coil is wound, a yoke part extending radially outward from the cylindrical part, and a claw-like magnetic pole part connected to the yoke part and surrounding the field coil. In this construction, the field coil is wound on an insulating bobbin and contacts the cylindrical part. An axial end surface of the bobbin is fixed to the yoke part through an adhesive agent. The stator has an iron core confronting the rotor and a multiple-layer winding mounted on the iron core. In this construction, an axial length (L1) of the iron core of the stator is set larger than an axial length (L2) of the cylindrical part. An axial thickness (X2) of the yoke part is set larger than a radial thickness (X1) of the claw-like magnetic pole part. The cylindrical part is separate from the yoke part and fixed to the yoke part with the cylindrical part in contact with the yoke part.
As described above, the axial length (L1) of the iron core of the stator is set larger than the axial length (L2) of the cylindrical part, and the axial thickness (X2) of the yoke part is set larger than the radial thickness (X1) of the claw-like magnetic pole part. In this construction, the temperature of the claw-like magnetic part greatly rises. As also described above, the cylindrical part is separate from the yoke part and fixed to the yoke part with the cylindrical part in contact with the yoke part. Thus, the contact surface of the cylindrical part and the yoke part serve as a thermal resistance, respectively. Thus, heat transmission from the yoke part to the cylindrical part is prevented. Consequently, temperature rise of the cylindrical part is reduced. Thereby, thermal stress is reduced, and thermal fatigue of adhesive material filled between the bobbin and the yoke part is reduced. Thus fixing the bobbin to the yoke part is accomplished without deteriorating the compactness of an AC generator and maintaining a high power output.
In another aspect of the invention, a gap is formed between a contact surface of the cylindrical part and that of the yoke part. Because the gap is formed between the contact surface of the cylindrical part and that of the yoke part, heat is prohibited from being transferred from the yoke part to the cylindrical part, thereby improving reliability of the generator.
In another aspect of the invention, an oxide film is formed on the contact surface of the cylindrical part and the yoke part. Therefore, prohibiting heat transfer from the yoke part to the cylindrical part.
In another aspect of the invention, the ratio of the axial length (L1) of the iron core of the stator to the axial length (L2) of the cylindrical part is set in the range of 1.25 to 1.75. Likewise, the ratio of the radial thickness (X1) of the claw-like magnetic pole part to the axial thickness (X2) of the yoke part is set in the range of 0.5-0.9. The output current is high because the ratios are set to the above-described range. Thus, the cylindrical part receives much heat from the claw-like magnetic pole part. Therefore, the generator effectively suppresses heat transfer.
In another aspect of the invention, in a plurality of respective spaced slots, the multiple-layer winding of the stator has pairs of conductive segments insulated from each other and arranged circumferentially without overlapping. One part of each pair forms an inner layer with respect to a depth direction of each of the slots. Likewise, the other pair forms an outer layer with respect to a depth direction. Outside the slots, the conductive segments extend to an end-surface side of the iron core of the stator to form coil ends by connecting the inner-layer and outer-layer in the slots in series to each other such that a group of the coil ends consisting of a plurality of repetition of the connection patterns is formed on the end-surface side of the iron core of the stator.
In this construction, the coil ends of the stator are prevented from overlapping each other. This allows the stator coil to be disposed in high density and with favorable ventilation. Thus, temperature rises of the stator coil are suppressed. Consequently, the amount of heat received by the claw-like magnetic pole from the stator is reduced, as well as the temperature rise of the cylindrical part. Accordingly, reliability of the AC generator is improved.
In another aspect of the invention, the rotor has 16 poles or more. Here, the generated electric frequency is high. Thus, eddy currents generate much heat. In addition, because the output current increases, the cylindrical part receives much heat from the claw-like magnetic pole part. Thus, the reliability of the generator can be improved owing to suppression of the heat transmission.
In another aspect of the invention, the rotor has a metal cooling fan mounted on both axial end surfaces thereof, thereby increasing heat transfer between the Lundel-type iron core and the cooling fan. Here, the configuration of the cooling fans are different from each other to vary exhaust capabilities thereof from each other and generate an axial flow inside the rotor. This allows the cooling fan to radiate heat favorably and an axial flow to be generated inside the rotor. Thereby, the temperature of the claw-like magnetic pole is reduced, and the AC generator is reliable.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.